1. Field of the Invention
The present invention generally relates to a distance/speed measuring method for measuring distances/speeds of detected targets (objects), and also a radar apparatus with employment of this distance/speed measuring method. More specifically, the present invention is directed to such a distance/speed measuring method capable of detecting the targets to measure relative distances and relative speeds between these targets by using a frequency-modulated continuous wave (FMCW), and further directed to a radar apparatus using this distances/speeds measuring method.
2. Description of the Related Art
Radar systems with employment of frequency-modulated continuous waves (FMCWs) are known in this technical field. In this known radar system, a beat frequency xe2x80x9cFupxe2x80x9d of a modulating frequency ascent period and another beat frequency xe2x80x9cFdnxe2x80x9d of a modulating frequency descent period are measured as to a target (object). In this case, the beat frequency xe2x80x9cFupxe2x80x9d of the modulating frequency ascent period and the beat frequency xe2x80x9cFdnxe2x80x9d of the modulating frequency descent period are expressed by the below-mentioned formulae (1) and (2):
[formula 1]
Fup=(2B/CT)xc2x7Rxe2x88x922/xcexxc2x7Vxe2x89xa1Frxe2x88x92Fvxe2x80x83xe2x80x83(1),
and
Fdn=xe2x88x92(2B/CT)xc2x7Rxe2x88x922/xcexxc2x7Vxe2x89xa1xe2x88x92Frxe2x88x92Fvxe2x80x83xe2x80x83(2).
As a result, a target relative distance xe2x80x9cRxe2x80x9d may be calculated from Fupxe2x88x92Fdn, and also a target relative speed xe2x80x9cVxe2x80x9d may be calculated from Fup+Fdn based upon the following formulae (3) to (6). It should be noted that in these formulae, symbol xe2x80x9cBxe2x80x9d shows a frequency modulation width; symbol xe2x80x9cCxe2x80x9d denotes a light velocity; symbol xe2x80x9cTxe2x80x9d represents a modulation period; symbol xe2x80x9cxcexxe2x80x9d shows a wavelength of a radar carrier wave; symbol xe2x80x9cFrxe2x80x9d denotes a distance frequency; and also symbol xe2x80x9cFvxe2x80x9d indicates a speed frequency:
[formula 2]
Fr=(Fupxe2x88x92Fdn)/2xe2x80x83xe2x80x83(3),
Fv=xe2x88x92(Fup+Fdn/2)xe2x80x83xe2x80x83(4),
R=(CT/2B)xc2x7Frxe2x80x83xe2x80x83(5),
and
V=(xcex/2)xc2x7Fvxe2x80x83xe2x80x83(6).
In such a case that the targets are present, combinations among beat frequencies xe2x80x9cFup(n)xe2x80x9d and the beat frequencies xe2x80x9cFdn(m)xe2x80x9d corresponding to the respective targets must be sought as to the beat frequencies Fup(i) (i=1 to N) which are detected in the modulating frequency ascent period, and further as to the beat frequencies Fdn (i) (i=1 to M) which are detected in the modulating frequency descent period.
For example, in Japanese Patent Application Laid-open No Hei-7-49378 and Japanese Patent Application Laid-open No. Hei7-55926, with respect to the frequency spectra of the reception signals in each of the periods, which are calculated so as to obtain the beat frequencies, the combinations of the beat frequencies are determined by utilizing the relative amounts as to the spectrum shape near peaks. Also, in Japanese Patent Application Lain-open No. Hei-7-77575, as to the frequency spectrum calculated to obtain beat frequencies, the combinations of the beat frequencies are determined by utilizing the magnitude order of the respective power levels of the peaks.
However, in any of the above-explained conventional radar systems, the combinations of the beat frequencies are erroneously determined due to the adverse influences, namely the variations in the moving (mobile) target signals, and also the unnecessary signal components such as the noise signals. As a result, these erroneous combinations are outputted as the false targets.
In this case, if such a discrimination can be carried out between a target which has no speed difference with respect to this radar system and may be observed as a stationary target from this radar system (will be simply abbreviated as a xe2x80x9cstationary targetxe2x80x9d hereinafter), and another target which has a speed difference with respect to the radar system and may be observed as a moving target (mobile object) from this radar system (will be simply abbreviated as a xe2x80x9cmoving targetxe2x80x9d hereinafter) among targets, then it is possible to reduce the appearance of the false targets caused by this erroneous combination.
For example, in Japanese Patent Application Laid-open No. 6-214017, Japanese Patent Application Laid-open No. 7-98376, and Japanese Patent Application Laid-open No. 7-191133, the power difference (see FIG. 9C) is calculated between the frequency spectrum of the modulating frequency ascent period (see FIG. 9A) and the frequency spectrum of the modulating frequency decent period (see FIG. 9B). Then, such stationary targets whose frequencies become equal to each other both in the modulating frequency ascent and descent periods are canceled, and only the moving targets are employed so as to be sought as the combinations.
However, when moving targets are employed as the targets to be sought, there is a certain case that a beat frequency (see FIG. 10A) in a modulating frequency ascent period of a certain moving target becomes equal to another beat frequency (See FIG. 10B) in a modulating frequency descent period of another moving target. Accordingly, there is a problem in that no final target detection output can be produced (see FIG. 10C).
The present invention has been made to solve the above-explained problem, and therefore, has an object to provide a target distance/speed measuring method, and a radar apparatus with employment of such a target distance/speed measuring method capable of measuring a distance and a speed between the respective targets while discriminating a moving target from a stationary target among targets.
A target distance/speed measuring method, according to an aspect of the present invention, is characterized in that in such a method for measuring distances and speeds of each of targets by employing a frequency-modulated continuous wave based upon a seeking result of combinations between beat frequencies detected for the targets in a modulating frequency ascent period and the beat frequencies detected in a modulating frequency descent period with respect to each of the targets, since moving targets are discriminated from stationary targets by using reference specific frequencies which correspond to a detection result with respect to a reference spectrum obtained as a calculation result between a frequency spectrum in a modulating frequency ascent period and a frequency spectrum in a modulating frequency descent period, the distances and the speeds of the respective targets are measured while the targets are discriminated as the moving targets and the stationary targets.
Also, the target distance/speed measuring method is characterized in that the specific frequencies equal to a detection result with respect to a reference spectrum which has been obtained during one preceding measuring operation of a presently measured modulating frequency ascent period and a presently measured modulating frequency descent period are utilized as a reference with respect to a repeated modulating frequency ascent period and a repeated modulating frequency descent period.
Also, the target distance/speed measuring method is characterized in that the reference spectrum is obtained from an absolute value of a power difference between a frequency spectrum of a modulating frequency ascent period and a frequency spectrum of a modulating frequency descent period.
A radar apparatus, according to another aspect of the present invention, is characterized by comprising: transmitting means for transmitting a transmission wave of a continuous wave whose modulating frequency ascends or descends to a target; receiving means for receiving a reception wave reflected from the target; beat signal producing means for producing a beat signal having a frequency corresponding to a frequency difference between the transmission wave and the reception wave reflected from the target; fast Fourier-transform processing means for acquiring a frequency spectrum of the beat signal; beat frequency detecting means for detecting the frequencies of the targets in both a modulating frequency ascent period and a modulating frequency descent period; discrimination target reference spectrum setting means for inputting thereinto a frequency spectrum of a modulating frequency ascent period and a frequency spectrum of a modulating frequency descent period so as to set a reference spectrum employed when the moving targets are discriminated from the stationary targets; discrimination target beat frequency detecting means for detecting the reference beat frequencies of either the moving targets or the stationary targets based upon the reference spectrum; combination seeking means for entering thereinto the beat frequencies of the target in the modulating frequency ascent period and the modulating frequency descent period from the beat frequency detecting means so as to form a combination seeking table, and for removing from a seeking object, a component other than the reference beat frequencies of either the moving targets or the stationary targets from the discrimination target beat frequency detecting means within the combination seeking table, to output; and distance/speed calculating means for calculating a distance of the target and a speed thereof based upon the beat frequency from the combination seeking means.
The radar apparatus is characterized by further comprising: a discrimination target reference beat frequency storing unit connected between the discrimination target beat frequency detecting means and the combination seeking means, for storing thereinto the reference beat frequencies of either the moving targets or the stationary targets detected by the discrimination target beat frequency detecting means; and for outputting to the combination seeking means the reference beat frequencies which have been obtained during one preceding measuring operation of a presently measured modulating frequency ascent period and a presently measured modulating frequency descent period with respect to a repeated modulating frequency ascent period and a repeated modulating frequency descent period.